1. Field of the Invention
The present invention relates to a sequential circuit that includes transistors having the same conductivity type, and a semiconductor device such as a semiconductor display device that includes the sequential circuit.
2. Description of the Related Art
Semiconductor display devices such as liquid crystal display devices and EL display devices preferably include transistors having the same conductivity type rather than CMOS transistors in order to reduce the cost of backplanes (circuit boards). Patent Documents 1 and 2 disclose techniques for forming a variety of circuits such as inverters and shift registers that are used in driver circuits of semiconductor display devices and are constituted of transistors having the same conductivity type.
In particular, for a semiconductor display device including oxide semiconductor transistors, a glass substrate of the fifth generation (1000 mm wide×1200 mm long, or 1100 mm wide×1300 mm long) or later can be used. Thus, such a semiconductor display device has advantages of high productivity and low cost. The oxide semiconductor transistors generally have the same conductivity type. Thus, in the case where the oxide semiconductor is used for a semiconductor display device, a driver circuit of the semiconductor display device includes transistors having the same conductivity type. Accordingly, the oxide semiconductor transistors are suitable for transistors included in a semiconductor display device.